This invention relates to devices for converting rotary motion to reciprocatory linear motion, and in particular to a device for using pneumatic or fluid pressure to produce said rotary motion, which in turn controls the incremental linear motion of a shaft.
Devices for converting rotary to linear motion and linear to rotary motion are well known. Several piston and crank configurations exist which translate either type of motion to its relative counterpart, and examples of machines using such a piston and crank--the internal combustion engine, steam locomotive, and sewing machine for example--are commonplace. Another design incorporates a screw shaft, in which the rotary motion of a fixed screw causes the linear movement of a screw shaft meshed with that fixed screw--the principle used in a log splitter or lathe. The rack and pinion is another simple device to connect the rotary motion of a pinion having gear teeth to the linear motion of a straight rack having corresponding gear teeth meshed with those of the pinion, as in a drill press or mechanical tuner.
Many systems use varying combinations of these simpler devices, or elements selected from those devices and assembled in more complex arrangements, to achieve the specific results required for a particular application. U.S. Pat. No. 4,436,163 discloses a mechanism for use in a power operated tool such as a jigsaw wherein the rotary motion produced by a motor is transferred to an output shaft and pinion which in turn meshes with a gear mounted on a second shaft connected to a crank disc. The crank disc has a bore to contain the ball-shaped head of an inclinable link that transmits the rotary motion of the crank disc to a reciprocating piston member.
While this mechanism displays one method of minimizing the torque exerted about the axis of the piston in a crank and piston arrangement that is being used to translate rotary to linear motion, it also serves to identify several of the drawbacks to crank and piston systems.
First, in order to operate efficiently and utilize a minimum of components, the axis of rotation of the crank and the linear path of the piston must be oriented in two different physical dimensions. Second, the movement of the piston may best be defined by the equations for a simple harmonic oscillator, and are therefore mathematically `nonlinear` in the sense that a unit change in the angle of the crank less than one half revolution will not produce a uniform change in the displacement of the piston. Third, the crank and piston arrangement is best applied in situations requiring rapid oscillation of the piston throughout a continuous circuit, rather than incremental movements of the piston in a random or alternating sequence of directions along its path. Fourth, obtaining an increase in the stroke length of the piston requires a corresponding increase in the radius of the crank disc which necessitates a physical alteration in the mechanical connection between the crank and piston. Fifth, if an alteration in the speed or power of the piston stroke is required, the crank speed must be changed by varying the motor speed or incorporating a mechanism such as a clutch to alter the gear ratio, thus requiring additional components and consuming increasingly larger volumes of space. Even with such modifications, it is difficult to attain instantaneous, alternating, or well defined changes in speed or power with such a system.
U.S. Pat. No. 4,489,792 discloses an adapter for a power drill designed to convert the rotational motion of the drill chuck into reciprocatory motion in order to produce a rotary drive, an impact force, or a combination of each. The device utilizes a fixed mode cam and a displaceable floating cam located in a housing between the drill motor and the chuck, and includes a rotating dial to select the desired output forces.
Although being relatively compact, providing mathematically `linear` motion in one direction, and aligning the axis of rotation of the rotary motion along the path of the linear motion, this device points out several drawbacks incumbent with cam-type systems.
First, the linear motion is again suitable only for rapid oscillations over a short stroke length, and cannot be used to provide incremental movement in a selected direction. Second, the reciprocatory motion is controlled by the motor in only one direction, and depends upon a recoil spring or hand pressure exerted by the user to complete each circuit and return the piston to its original position. Third, the source of the rotary motion--in this case the motor and rotary drive shaft--is displaced relative to the path of the linear motion.
U.S. Pat. No. 3,323,160 discloses a surface treating device designed to accomplish results analogous to those of the previous device, yet capable of correspondingly slower oscillations with a comparatively heavier load, by using a variation of the piston and crank discussed previously.
U.S. Pat. No. 3,323,382 discloses a precision linear actuator of a type now commonly referred to as a digital linear actuator. These actuators use an electric stepper motor with an internally threaded rotor coupled to a correspondingly threaded lead screw shaft, so that by energizing the coils of the stepper motor in the proper sequence the threaded shaft may be moved outward or inward relative to the rotor.
Such actuators may be used to position the path of linear motion of the shaft in increments along a region overlapping and contiguous with the axis of rotation of the rotary motion. However, because they require electric motors to provide their drive power, and a complex array of electronic circuitry to time the energizing of the coils and control the incremental movement of the shaft, these actuators are not suitable for use in those existing systems which have pneumatic or fluid control mechanisms and utilize cylinders driven by pneumatic or fluid pressure to obtain the desired linear motion. Although these fluid power cylinders lack the accurate control over the incremental linear movement which may be attained with digital linear actuators--because the pressurized gases or fluids remain compressible to some degree, are subject to temperature fluctuations over extended time periods, and conform to somewhat variable relationships relating the pressure and volume of the fluid to the linear displacement of the cylinder piston--they continue to be instrumental components in many systems because the physical dimensions of the cylinders and the expense or complexity of replacing the control systems mitigate against adopting a precision linear actuator.
The pneumatic or fluid cylinders described above which translate fluid pressure directly into linear motion of a rod may be categorized in several ways: single-acting, cushioned, double-acting, ram, spring return, threaded head, double-end rod, telescoping, multiposition, diaphragm, rotating, slotted, or rodless. Because of their many applications, a uniform system of dimensions, design specifications, and standards has been established to govern these cylinders through the National Fluid Power Association (NFPA). Consequently, engineers may rely upon the interchangability and compatibility of various fluid cylinders which are produced in accordance with the NFPA guidelines.
U.S. Pat. No. 4,592,430 discloses a power tool for anchoring threaded fasteners which is capable of delivering reversible rotary motion and is driven by fluid pressure. The tool utilizes an eccentric cylinder and floating vane configuration found in pumps and fluid motors, but produces only rotary motion which is subsequently transferred to a drive shaft and bit holder.
This device displays the inordinately complex array of control elements which must be incorporated into a reversible valve mechanism for such a fluid operated tool, without including any elements to translate the rotary motion of the shaft into linear motion.
One other known device to convert linear motion produced by pressure into rotary motion, in which the linear motion is reciprocating and in which the rotating shaft is, at times, received within the reciprocating housing, is the "dixie drill." The dixie drill, however, cannot effectively convert rotational motion to distinct and incremental linear motion, and is not adapted for use with pneumatic or fluid pressure systems.